The heart includes a natural electrical system that generates electrical impulses, which cause the heart to contract. When functioning properly, the electrical impulses generated in the heart cause the heart to beat in a coordinated fashion. If, however, the heart's electrical system malfunctions, the heart will not beat in a coordinated fashion, which can result in various types of cardiac arrhythmias.
Cardiac arrhythmias can be classified into two general categories, bradycardia and tachycardia. Bradycardia is an abnormally slow or unsteady heart rate, whereas tachycardia is an abnormally fast heart rate. Tachycardia arrhythmias may be further classified into two general subcategories, supraventricular tachycardia and ventricular tachycardia. There are various types of supraventricular tachycardias, including atrial fibrillation (AF) and atrial flutter. There are also various types of ventricular tachycardias (VT), which include “pulseless VT,” “VT with pulse,” and ventricular fibrillation (VF).
Many cardiac arrhythmias can be treated by delivering an electrical pulse to, or in the vicinity of, the heart. The magnitude, duration, and number of electrical pulses that are delivered may, in many instances, depend upon the type of cardiac arrhythmia being experienced. For example, defibrillation pulses, which are relatively high in magnitude, may be used to treat, for example, VF, and pulseless VT arrhythmias, whereas pacing pulses, which are of a relatively lesser magnitude than defibrillation pulses, may be used to treat, for example, VT with pulse, AF, and atrial flutter arrhythmias, and bradycardia arrhythmias.
Various types of implantable devices are available to generate and deliver the various types of electrical pulses described above. However, not all individuals that experience a cardiac arrhythmia have such a device implanted. Thus, various types of external defibrillators have been designed and manufactured, including both manual and automated external defibrillators (AEDs). Most external defibrillators, both manual and automatic, include electrical pulse generation circuitry and a pair of pulse delivery electrodes. The electrical pulse generation circuitry generates a therapy pulse that may be applied to a patient via the pair of pulse delivery electrodes, when the pulse delivery electrodes are positioned on the patient's chest. With a manual defibrillator, the energy level of the therapy pulse applied to a patient may be manually adjusted. With an AED, the energy level of a therapy pulse may preferably be adjusted automatically, though in some cases the energy level may be also adjusted manually. In either case, the energy level of the therapy pulse to be applied will depend on whether the patient needs to receive, for example, a defibrillation pulse or one or more pacing pulses.
No matter the type of therapy pulse a defibrillator delivers to a patient, it is highly desirable that the patient's ECG be monitored after the therapy pulse is delivered. That way a meaningful determination can be made as to whether the patient needs one or more subsequent therapy pulses, and the energy level at which each subsequent pulse should be delivered. Generally, when an external defibrillator is being used, a patient's ECG is monitored in one of two ways. If the external defibrillator is a manual defibrillator, separate ECG monitoring electrodes may be applied to the patient. If the external defibrillator is an AED, the patient's ECG may be monitored via the pulse delivery electrodes or, in some instances, using separate ECG monitoring electrodes. It would be preferable if a patient's ECG could be monitored via the pulse delivery electrodes, no matter the type of defibrillator being used or the energy level of the defibrillator pulse being applied; however, for at least the following reasons, this is presently not practical.
Cardiac therapy pulses can range from a few hundred volts (for pacing pulses) up to a few thousand volts (for defibrillation pulses). Thus, when a cardiac therapy pulse is delivered to a patient, the electrodes accumulate an electrical charge. This electrical charge gradually decays after the pulse is delivered, but this decay can take from several hundreds of milliseconds (for a pacing pulse) up to several seconds (for a defibrillation pulse). An ECG signal, which may be only a few millivolts in magnitude, may be generated approximately 100 milliseconds (or less) after a cardiac therapy pulse is delivered. However, because of the relatively large electrical charge that has accumulated on the pulse delivery electrodes, this ECG signal may not be detectable via the pulse delivery electrodes until the accumulated charge has sufficiently decayed.
Thus, manual defibrillators are presently not useful for administering periodic transthoracic pacing pulses to a patient, unless additional ECG monitoring leads are also applied to the patient. Moreover, present AEDs that are not equipped with separate ECG monitoring leads are not capable of accurately detecting ECG signals after a therapy pulse has been delivered, until the accumulated charge on the pulse delivery electrodes has sufficiently decayed. The time it takes for the accumulated charge to decay, can result in an undesirable delay in the delivery of a subsequent therapy pulse, or can delay a decision to administer cardiopulmonary resuscitation (CPR). Although additional ECG monitoring electrodes can, and sometimes are, provided with manual defibrillators and AEDs, it would be preferable if such leads could be eliminated, most notably for AEDs, since the medical skill level of persons operating AEDs may not be high.
Hence, there is a need for a system and method of monitoring ECG signals during cardiac therapy pulse delivery that does not rely on additional ECG monitoring leads and/or allows detection of ECG signals substantially immediately after a cardiac therapy pulse has been delivered without the use of additional ECG monitoring leads. The present invention addresses these needs. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.